Tephrostratigraphy, Magnetostratigraphy and Geochronology of Some Early and Middle Pleistocene Deposits in New Zealand

Abstract

Numerous early Pleistocene silicic tephras are exposed in long sedimentary sequences in the East Coast and Wanganui basin regions in southern North Island of New Zealand, some 150-250 km south of the Taupo Volcanic Zone. They provide time planes that can be correlated between different facies and basins. Individual tephras can often be distinguished on the basis of major and trace element glass chemistry, and Fe-Ti oxide composition. Approximately 51 different eruptive events may be recorded in the interval from ca. 1.7 Ma to 0.5 Ma. Early Pleistocene tephras in deep-sea sediments of the Southern Pacific Ocean at latitudes >60°S were previously considered to have been sourced in the TVZ. However, their alkalic compositions are compatible only with volcanoes of Western Antarctica and the Ross Sea region. Most of the tephras examined here are reworked, and many have been emplaced as catastrophic flood deposits in overbank settings of braid plains in the East Coast region. Their mode of emplacement and the presence of ignimbrites in the sequences indicate early Pleistocene transport routes through the site of the present main Axial Ranges, and suggest substantial tectonic uplift in the last 0.8 Ma. Long sequences spanning the Jaramillo Subchron (0.99-1.07 Ma) and older Matuyama Chron are recognised at Mangatewaiiti and Mangatewainui in the East Coast region, and Rewa Hill in the Rangitikei Valley. Numerical age control is provided by 40Ar/39Ar single crystal laser fusion ages from plagioclase in key tephra horizons. This new chronology indicates the tephras are nearly twice as old as several previous studies have suggested, thus requiring a major revision of the New Zealand Pleistocene stratigraphy. By integrating isotopic, paleomagnetic and geochemical data, 3 widespread tephras can be correlated between basins of the East Coast and Wanganui: Pakihikura Tephra (ca. 1.6 Ma), Potaka Tephra (1.00 Ma), and Kaukatea Tephra (ca. 1 Ma). These tephras and others provide a chronological framework for much of the early Pleistocene in southern North Island. Potaka Tephra is particularly widespread, allowing correlation between marine strata of the Castlecliffian (local early Pleistocene stage) type section at the Wanganui coast, and marine strata elsewhere in the Wanganui basin, as well as with fluvial and lacustrine strata in the East Coast. The tephra occurs as an ignimbrite and as a catastrophic flood deposit in the East Coast and as a fallout ash in North Canterbury, South Island (ca. 600 km from source). Potaka Tephra (normal polarity) and Kaukatea Tephra (reversed polarity) bracket the top of the Jaramillo Subchron and constrain its age to ca. 1 Ma. This is in accord with the astronomical calibration of the Pleistocene geomagnetic time scale, but older than previous determinations using the 'chronogram’ method on K-Ar data. The precise source vents for the distal early Pleistocene tephras are uncertain, however their ages indicates they are coeval with dated proximal ignimbrite sheets from the Mangakino Caldera in the SW part of TVZ. The large number of distal tephras would imply a greater frequency of eruptions from this source than previously expected.